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Half Life help

  1. Feb 27, 2005 #1
    At a certain temperature the reaction

    2N2O5 --> 4NO2 + O2

    is a first order reaction with t1/2 = 153.0 sec

    What is the rate constant for this reaction (1/sec) ?

    Enter a numeric answer only, do not include units in your answer

    Ok I know the equation is t1/2= ln2/k

    so isnt k the rate constant, and wouldnt the answer be ln2/153 sec= .0045??
  2. jcsd
  3. Feb 27, 2005 #2


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    Staff: Mentor

    The first order equation is -

    [itex]\frac{d [A]}{dt} = - k [A] [/itex]

    where k is the rate constant.

    The solution, as you correctly mentioned, is -

    [itex]ln \frac{[A]}{[A_o]} = - k t[/itex], where [A] is the concentration at time t, and Ao is the initial concentration, at t=0.

    The definition of half-life is-

    [itex]ln \frac{[A_o]/2}{[A_o]} = - k t_{1/2}[/itex], or

    ln 2 = k t1/2
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